1. Field of the Invention
This invention relates to the non-destructive testing of ferromagnetic materials and, in particular, to means for reducing the sensitivity of test apparatus to variations in relative velocity of an object under test and a probe head supplying the test apparatus.
Many devices for the non-destructive testing of ferromagnetic materials rely on inductive sensors such as pickup coils to detect local differences in magnetic field and produce an electrical signal dependent thereon. These signals may be recorded, fed on to some form of data processing apparatus or simply displayed on an indicator device. However, the differences in magnetic field which they represent are, in turn, indicative of local variations in the object under test. These variations may result from flaws or profile irregularities or inhomogeneities in the materials of which the object is constructed. The voltage induced in an inductive pickup coil is given by EQU V=k(d.phi./dt)
where d.phi./dt is the rate of change of magnetic field and k is a constant which depends on the geometry and magnetic properties of the pickup coil.
A major application of methods of non-destructive testing of ferromagnetic materials is the inspection of pipelines which are buried below the ground or submerged in water and are used to transport gases and liquids. Inspection vehicles known as pigs may be driven by fluid pressure and traverse the pipeline with the fluid. These inspection pigs carry test apparatus which includes sensors to investigate properties of the pipeline. They may carry recording equipment or use telemetry to transmit the test measurements to a remote location. A typical method and apparatus for the non-destructive testing of ferromagnetic pipelines is described in our co-pending British application No. 3189/77.
2. Prior Art
When carrying out measurements on test objects, it is frequently convenient to preprocess the data before transmitting it to a remote location or recording it. This may be necessary because the facility for recording or transmitting the data has only a limited bandwidth or dynamic range, or it may be simply to filter out unwanted data for ease of evaluation and interpretation. One such system is the use of a trigger threshold as is described in DAS No. 2,704,132, where the method is employed for statistical sampling of an ultrasonic signal. Another system is to filter the transducer signals so that only those characteristic of the parameters of interest are allowed to pass; this method is followed in instruments for investigating the surface profiles produced by machining. Yet another system is to perform an autocorrelation process on signals produced by a measuring transducer; one form of seismological surveying uses this technique.